High-intensity focused ultrasound (HIFU) ablation of the prostate has conventionally been done through the rectal wall with a transrectal probe. Alternatively, ablation can also be done through the wall of the urethra using a transurethral probe. The transurethral approach has several safety-related advantages as compared to the transrectal approach. Because the location of the urethra is known (probe within urethra), it is easier to avoid unintentional thermal damage to the urethra that might increase the risk of incontinency. Moreover, since there is no sonication through the rectal wall, the risk of damaging this sensitive structure is also significantly reduced. The main disadvantage of the transurethral approach is that the space available for the transducer is substantially reduced as compared to the transrectal approach. This basically limits the catheter transducer design to one dimensional phased arrays if employing conventional piezoceramic or piezocomposite transducers, thus also limiting the possible sonication methods that can be used. For practical reasons, traditional transducers are ridged with a relative low number of large elements, arranged in a row, i.e. a linear (one dimensional) array.
In Ergun et. al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, pp. 2242-2258 (2005) the fabrication and use of capacitive micromachined ultrasonic transducers for imaging is reviewed.
The US-patent application US 2008/0221448 mentions a catheter with an annular array of capacitive micromachined ultrasound transducers for real time forward looking acoustic imaging with a HIFU device for tissue ablation.